Sound translating apparatus



Sept. 7, 1954 H. F. OLSON 2,688,373 SOUND TRANSLTING APPARATUS Filed Mayl 1951 VA Ms CAS Cm: 1Q/f Q/a A K ,5 Pl

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fifa/[Nay ENTOR ULSDN ATTORNEY Patented Sept. 7, 1954 SOUND TR-AN SLATING APPARATUS Harry F. Olson, Princeton, N. J., assgnor to RadioCorporation of America, a corporation of Dela- Ware Application May 1,1951, Serial No. 223,865

11 Claims. l

The present invention relates to sound translating apparatus, and moreparticularly to an improved loudspeaker and method for convertingelectrical energy into acoustical energy.

An important consideration in providing high quality sound reproductionis that the sound translating apparatus have uniform acoustical poweroutput over a wide range of lsound frequencies. Although many excellentsystems have been proposed in the past, the sound reproducing quality ofrelatively small apparatus in use today,

particularly apparatus of the kind used in the home or in places wherespace is relatively limited, is not too good. In many home instruments,in order to obtain adequate low frequency response, a large open backcabinet is used. Other acoustical systems employ relatively large,closed cabinets, having a base reflex opening for accentuating low.frequency acoustical power output. In others, a plurality ofloudspeakers are mounted in a single cabinet to provide suitablecoverage over a wide frequency range.

The use of a closed back loudspeaker cabinet is, however, generallyaccepted as superior to that of the open back cabinet for soundreproduction,

`since the later type of cabinet exhibits a serious resonance whichaccentuates the acoustical power output at the particular resonantfrequency. In addition, the low frequency acoustical power output of theopen back cabinet below the resonant frequency of the loudspeaker ishighly attenuated.

It is a primary object of the present invention to provide an improvedsound translating apparatus the dimensions of which are relatively smalland the acoustical power output of which extends over a wide range ofuseful sound frequencies.

It is also an object of the present invention to provide an improvedsound translating device of relatively small dimensions the acousticalpower output frequency characteristic of which is comparable to that oflarger instruments having excellent sound reproducing qualities.

Another object of the present invention is to improve the soundreproducing qualities of a loudspeaker mounted in a relatively small,closed back cabinet.

Still another object of the present invention is to provide a method ofoperating a loudspeaker mounted in a closed cabinet which will enhancethe sound reproducing qualities thereof.

A further object of the present invention is to provide a method ofoperating a loudspeaker mounted in a closed cabinet which will extendthe acoustical power output frequency range thereof.

It is another object of the present invention to provide a soundreproducing apparatus having a closed back cabinet of relatively smalldimensions with excellent fidelity over a wide frequency range, andwhich is also highly efficient in use.

In accordance with the present invention, the sound reproducingapparatus comprises a cabinet structure arranged to provide twoacoustically closed chambers. An opening is provided in a wall of thecabinet, between one of the chambers and the exterior of the cabinet. Aloudspeaker diaphragm is disposed over this opening and forms a closuretherefor. A sound transmitting passage is provided between the chambersand a second r loudspeaker diaphragm is mounted in a manner to form aclosure therefor. The two loudspeakers are connected to the output stageof a conventional Vaudio frequency amplifier. The loudspeaker mountedbetween the chambers is adapted particularly to reproduce mostefficiently low audio frequency sound waves whereby mechanical couplingbetween the diaphragms through the air medium common to both diaphragmsserves to assist in driving the other diaphragm in the low frequencyrange thereby enhancing the sound reproducing quality of the apparatusby extending the overall acoustical power output frequency range.

The novel features characteristic of the present invention, as well asadditional objects and advantages thereof, will be understood betterfrom the following detailed description when read in connection with theaccompanying drawing, in which Figure l is a side view, partly brokenaway, of a single loudspeaker mounted in a closed back cabinet,

Figure 2 is a wiring diagram of an electrical circuit corresponding tothe acoustical network of the vibrating system of the apparatus shown inFigure 1,

Figure 3 shows two curves which indicate the acoustical power outputfrequency characteristics of a loudspeaker mounted in closed backcabinets of diierent volumes,

Figure 4 is a side View, partly broken away, of a sound reproducingapparatus in accordance with one embodiment of the present invention,

Figure 5 is a wiring diagram of an electrical network corresponding tothe acoustical network of the vibrating system of the apparatus shown inFigure 4:

Figure 6 shows two curves, one of which indicates the electricalimpedance frequency characteristic of one of the loudspeakers shown inFig- 3 uie 4, and the other of which shows the electrical impedancefrequency characteristic of the other loudspeaker,

Figure 7 shows two curves representing a comparison of the acousticalpower output frequency characteristics of the apparatus shown in Figures1 and 4, both cabinets having the same volume,

Figure 8 is a sectional view of a second embodiment of sound reproducingapparatus in accordance with the present invention, and

Figure 9 is a-wiring diagram of an electrical network depicting theelectrical impedance of the two voice coils of the systems shown inFigures 4 and 8.

Referring more particularly to the drawing, wherein similar referencecharacters designate corresponding parts throughout, there is shown, inFigure l, for comparison purposes, a closed back cabinet l in which aloudspeaker 3 is mounted with the diaphragm thereof disposed behind anopening 'I provided in a wall 9 of the cabinet so as to form a closurefor the opening. The acoustical network for this type of enclosedloudspeaker cabinet may be represented by yan electrical circuitdiagram, as shown in Figure 2 of the drawing, wherein:

p1=the driving pressure of the loudspeaker, which may be calculated fromthe equation:

fm1 Pi- A1 (l) where and where Ms=inertance of the cone diaphragm, thevoice coil and the air load on the cone diaphragm of the loudspeaker;

CAs=acoustical capacitance of the cone diaphragm suspension system;

CAQ-:acoustical capacitance of the cabinet, as calculated from theequation:

where V=volume of the cabinet; p.=d.ns'1ty of air; and cvelocity of sound.

`The acoustical volume current in the system is given by the equation:

j(l: l p1 TA +7wMs-C'As-.wCAC

vwhere yew/1 1' cp=21rf f :frequency i The acoustical power output ofthe system is given by the equation:

P 7 AX 12 (4) Referring to the Equations 3 and 4- above, it will be seenthat the acoustical power output lin the low frequency range isinfluenced by the cabinet volume. As the cabinet volume is decreased,the low frequency cut-off in acoustical power output is increased infrequency.

If, for example, a loudspeaker having a twelve inch diameter diaphragmwith a resonant frequency of 70 cycles per second is mounted in a closedback cabinet l, in the manner shown in Figure 1, the acoustical poweroutput frequency characteristic of the apparatus using cabinets havingvolumes of 5 and 2 cubic feet, respectively, may be represented bycurves as shown in Figure 3. In this family of curves, curve Arepresents the acoustical `power ouput frequency characteristic for thetwelve inch loudspeaker mounted in a ve cubic foot cabinet, and curve Brepresents the acoustical power output frequency characteristic for thatsame loudspeaker mounted in a two cubic foot cabinet. A comparison ofthese two curves shows that, in the case of the ve cubic foot cabinet,the resonant frequency of vthe loudspeaker is approximately cycles persecond, whereas, in the case of the two cubic foot cabinet, the resonantfrequency is approximately cycles per second. A comparison of these twocurves shows that, the smaller the cabinet cavity the higher theresonant frequency of `the loudspeaker. In either of these cases, theresonant frequency is too high and the acoustical power output falls offrapidly below the resonant frequency thereby reducing Vthe output in thelow frequency range, a condition unsuitable for excellent soundreproduction.

On the other hand, if a loudspeaker having a lower resonant frequencywere used, for example, one having a 50 cycle per second resonantfrequency alone, the resonant frequencies of that loudspeaker mounted incabinets of 5 and 2 cubic feet volumes, would be respectively, 86 and120 cycles per second. However, experience has shown that a twelve inchloudspeaker with a 50 cycle resonant frequency is not only diiicult toconstructy but it is also dimcult to. maintain uniform acoustical poweroutput in the mid-frequency range due to break up of the`flexiblesuspension system for the diaphragm.

The current trend being toward the use of smaller cabinets. for soundtranslating instruments, the present invention is particularly concernedwith the improvement in low frequency acoustical power output ofconventional. types of loudspeakers mounted in closed back cabinets ofrelatively small volume. For average use, a tolerable cabinet would havea volume of approximatelyV two cubic feet.

In accordance with one embodiment of the present invention, improvedsound reproducing quality of relatively small apparatus is obtained byemploying an acoustically closed cabinet or walled enclosure lfhaving anopening l'l through awall i9 thereof. A partition 2 Il having an opening2.3 therein is mounted with-in the cabinet so that it divides theinterior into two chambers 25 21.

A sound reproducer comprisingz a loudspeaker 29 isA mounted with thediaphragm 3L! thereof covering the wall opening lil-'so that it formswith the cabinet wal-l a closure forY that opening. A second loudspeaker33. is mounted with the diaphragm 35 thereof covering the partitionopening 23 so thatit forms with the-'partitiona closure for f thatopening. Thus, the loudspeaker diaphragms together with the enclosurewalls and the partition provide two acoustically closed chambers.

In one sound reproducing apparatus, made for experimental use, a cabinethaving a volume of two cubic feet is provided. A loudspeaker 29 having atwelve'inch diameter cone diaphragm 3| and a resonant frequency of 70cycles per second is mounted behind the opening I1 connecting thechamber 25 with the exterior of the cabinet. A second loudspeaker 33having a ve inch cone diaphragm is mounted with its diaphragm 35covering the partition opening 23. The partition ZI is arranged in thecabinet I5 so that an acoustically closed chamber 25 of one-half cubicfoot volume is provided behind the larger diaphragm. The other chamber21 has a volume of one and one-half cubic feet. The respectiveelectro-mechanical driving means for the loudspeakers 29, 33 areconnected electrically in series with a common source 'of energyprovided by a conventional audio frequency amplier (not shown). Ifdesired', the electro-mechanical driving means may be connectedelectrically in parallel with the source of energy. The acousticalnetwork for this arrangement is shown by an electrical circuit diagram,such as that illustrated in Figure 5 of the drawing, wherein:

p1=driving pressure of the large loudspeaker, as

determined from the equation,

where B1=ux density in the air gap in the large loudspeaker;

l1=length of the conductor in the voice coil of the large loudspeaker;

i1=current in the voice coil of the large loudspeaker; and

A1=area of the cone diaphragm of the large loudspeaker;

p2=driving pressure of the small loudspeaker, as

determined from the equation:

where B2=fiux density in the air gap in the small loudspeaker;

l2=length of the conductor in the voice coil of the small loudspeaker;

i2=current in the voice coil of the small loudspeaker; and

A2=area of the cone diaphragm of the small loudspeaker;

m=acoustical radiation resistance presented to the cone diaphragm of thelarge loudspeaker;

Ms1=inertance of the cone diaphragm, the voice coil and the air load onthe diaphragm of the large loudspeaker;

Ms2=inertance of the cone diaphragm, the voice coil and the air load onthe diaphgram of the small loudspeaker;

CAs1=acoustical capacitance of the diaphragm suspension of the largeloudspeaker;

CAs2=acoustical capacitance of the diaphragm suspension of the smallloudspeaker;

Casa=acoustical capacitance of the volume of air in the small chambercommon to both loudspeakers, as determined from the equation V3 Cass-pc2 Where The volume current in branch I of the electrical circuit isgiven by the equation:

Where D 'ZAiZAz -I- ZAiZAs -iZAzZAa v(The acoustical impedances ZM, ZMand Zas refer to the acoutical impedances of the branches I, 2 and 3respectively of the network of Figure 5.)

The acoustical impedance of branch I of Figure 5 is given by theequation:

J', wC'Asi branch 2 of Figure The acoustical impedance of 5 is given bythe equation:

wC'Asa (11) The acoustical impedance of branch 3 of Figure 5 is given bythe equation: I

J' J' A3 MM2 wCAsz wC'Asi (12) The volume current in branch 3 is givenby the equation:

Since the two loudspeakers are connected in series, i1=i2, and

P1 BiZiAz -==K P2 Bzlzl where K: constant.

The acoustical impedance at p1 is given by the equation:

D Zai ZAK) 15) ZA 2 l-ZA a The acoustical impedance at p2 is given bythe equation:

D Z :m

2 zA2+zA1+KcA2 (16) The electrical circuit of the Voice coils of thelarge and small loudspeaker connected in series is shown in Figure 9.The electrical impedance and the quantities in this circuit will now bedened. In the electrical circuit, the electrica-l impedacef oft the;two.. loudspeakers: connected--K in seriesuisgiverrby the` equation; 1

ZEMz--motional electrical impedance of the voice coil of the smallloudspeaker. The motional electrical impedance of the voice coil ofthesmall loudspeaker isy given by theequation:

eine (19) The;t current. .through y the voicey ycoils..is given-.by

tnaequation:

where e=voltage applied.to-the=.voice coils;

The acoustical power output is the power output deliveredzbytthediaphragm of the large loudspeaker, and isgivenby. the equation:

PwncV (2r) Employing llquationsi)` .toA 2l inclusive it is possible todetermine-theacoustical power output of theisystem.. The'above analysisshows. thatit is possible to eirtendtheflowl frequencyk rangefby the useof a second loudspeaker.

Several modelsoflthissystem have been built. As a typicalexample; infone model, the diameter of thee cone; diaphragm of the largeradiatingloudspeaker was 12 inches and the,dame.ter. of the cone diaphragmof4Vthe, small driving loudspeaker wasinchesl. The massIof the voice coil ofthe small loudspeaker was about twice that of thelarge.- loudspeaker..Theiux densityv inA the air gap was 12,000 gausses as comparedto 10,000for the large loudspeaker. The resonant frequency of thesmallfloudspeaker was 50 cycles per second and the largeloudspeaker 80cycles per second. rf'he total cabinet volume-was 2f cubic feet. The.volume behind the small loudspeaker Was ll/g cubic feet: Thevolurneb'eiweerrthev two loudspeakers was less than 1/2 cubic foot: E1n'-ploying this system, an examination of the acoustical circuit shown in;Figure 5 and Equations 9 to inclusive, discloses, that the load on theback ofthe large loudspeaker diaphragm is Very small in' the region"from' 50 to 300` cycles' perfsecond. Thus, the acoustical power outputconditions; as far as the large loudspeaker is, concerned, arepractically the same. asif. the large loudspeaker were mounted in a..cabinet having a very large volume. The electricalz impedanceA frequencycharacteristic of the largey speaker valone is shown by' curve A inFigure 6i |The electrical impedance frequency characteristic of thesmalll loudspeaker mounted in al cabinet oi one and one half cubic 8f.feet.. is shown. by curve Br liligure.v 6. Iny the regioniabove30.0,cyclesfper. second,vthe.acoustifcal impedance-o thechamber of onehalf, cubic foot volume is; smallcompared to the acousticall impedancevof; the large-diaphragm and, therefore, does-notinuence theacousticalpower output. It. will beyobserved fromv the curves shown` inEiguree` that above. 30,0 cyclesl per secondtheelectr-ical. impedance,of, the. small loudspeaker is very smallrcomparedtotheelectricalimpedance ofthe largefloudspeaker. As av result, since thetwo loudspeakers are; connected in seriesv practicallyr allatlfieinput`electrical power sfed to the large loudspeaker in the region above 300cycles. Under the above-.conditionsthelarge loudspeaker operates aloneand; iny the,Y conventional manner above :300;cycles per second.

The acoustical power output frequency. char,- acteristic; for,thecombination or closely coupled loudspeakers. mounted in a cabinet oftwo cubic feet volume: is shown by,A curve A in Figure '7. For.-comparison, purposes, the acousticaly power outputmharacteristicfor the12 inch loudspeaker mountedzalonein\a12cubic foot cabinet, as shown byAcurve-.Bin Figure 3, is slperimposed on curve Aeinrigure 7. It will beseen fromacomparison of these two.curves that there is considerableextension of the low frequency acoustical power output. This is`explained` by the introduction of the smaller loudspeakerI and: couplingit mechanically with the large loudspeaker through the enclosed volumeof airinlthe small chamber 25. Thus, in the region below 300 cycles, thesmall loudspeaker, which is adapted to reproduce most eiiciently soundwaves of relatively low audio frequencies in the desired range offrequencies, functions as. a. supplementary driving element for thelarge loudspeaker, which is adaptedto reproduce. eiciently. sound wavesover that, entire range. Eflectively, the sound pressure developed by.the small. loudspeaker. reduces the acoustical impedance presented tothe back of the large loudspeaker, over thatwhich would exist if'thelarge loudspeaker operated directly into, the entire cabinet.. Intheexample referredito above, over the frequency range, from` 50 to 300cycles per second, the acousticall impedance presented to the back ofthe large loudspeaker is practically the same as that of a.. very largecabinet.

From the foregoing description, it will be apparent that the presentinvention provides animproved sound reproducing system for completelyclosed loudspeaker cabinets of relatively small dimensions..

It will be obvious to persons skilledinA the. art that: variousmodicationsand changes in the sound reproducing apparatus shown anddescribedV herein ,are-possiblewithin the spirit of the presentinvention. For example, as. shown in Figure 8;the partitionzl may bedisposed diagonally of the interior of the cabinet. On the other hand,ytwo separate, acoustically closed cabinets may be provided; with meansfor transmitting sound" from one or'the loudspeakers mounted inonecabinet tothe chamberprovidedlby the other cabinet. Other changesoflike nature will, or course, readily suggest themselves. Therefore, itis-desiredithatthe particularforms of the present invention shownyanddescribed herein shall' be considered as illustrative andnotaslimiting.

is claimed is:

l. Sound reproducing apparatus comprising awallecl4 enclosure;meansdividing the interior of said enclosure into separate acousticallyclosed chambers, an exterior wall of said enclosure having an openingtherein connecting one of said chambers with the exterior of saidenclosure, said dividing means including an opening therein deiining asound passage between said chambers, sound reproducing means including adiaphragm mounted over each of said openings and forming a closuretherefor, and electro-mechanical driving means connected to each of saiddiaphragms.

2. The invention as dened in claim l wherein said sound reproducingmeans comprises a plurality of closely coupled loudspeakers, one of saidloudspeakers being adapted to reproduce efiiciently sound waves over theentire range of desired audio frequencies and the other of saidloudspeakers being adapted to reproduce most eniciently sound waves ofrelatively low audio frequency in said range.

3. The invention as defined in claim 1 wherein the volume of the chambercommon to both said diaphragms is substantially smaller than the volumef the other of said chambers.

4. The invention as defined in claim 1 wherein said electro-mechanicaldriving means for each of said diaphragms is connected to a. commonsource of electrical energy.

5. The invention as dened in claim 1 wherein one of said diaphragms islarger in area than the other of said diaphragms.

6. Sound reproducing apparatus comprising a walled enclosure, meansincluding a partition having an opening, a loudspeaker diaphragmcovering said opening, said last named means dividing the interior ofsaid enclosure into two acoustically closed chambers, one wall of saidenclosure having an opening therein connecting one of said chambers withthe exterior of said enclosure, and a second loudspeaker diaphragmmounted over said opening.

7. Sound reproducing apparatus comprising walled enclosures definingfirst and second acoustically closed chambers, said first and secondchambers having a common wall, the rst one of said chambers having anopening in an exterior wall thereof, a loudspeaker including a diaphragmmounted over said opening and forming a closure therefor, an opening insaid common wall, a second loudspeaker including a diaphragm mounted insaid opening in said common wall and forming a closure therefor.

8. Sound reproducing apparatus as defined in claim 7 wherein saidenclosures are dimensioned to provide acoustically closed chambers ofdifferent sizes.

9. Sound reproducing apparatus as defined in claim 8 wherein theenclosure having an opening in the exterior wall thereof is dimensionedto provide the smaller one of said chambers.

10. Sound reproducing apparatus as dened in claim 9 wherein one of saidloudspeakers is adapted to reproduce most efciently low audio frequencysound waves, said one of said loudspeakers having its said diaphragmmounted over the opening in the enclosure having the smaller one of saidchambers.

11. Sound reproducing apparatus as defined in claim 10 wherein saidloudspeakers are connected electrically in series.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,969,704 DAlton Aug. 7, 1934 1,988,250 Olson Jan. 15, 19352,114,680 Goldsmith Apr. 19, 1938 2,143,175 Waite Jan. 10, 19392,217,279 Karns Oct. 8, 1940 2,539,327 Reid et al Jan. 23, 1951 FOREIGNPATENTS Number Country Date 585.963 France Mar. 12. 1925

